Direct Restorative Dental Materials
Loss of tooth structure to caries or other processes usually needs restoration of a small portion of tooth structure that is defective. This can be accomplished relatively easily by designing a tooth preparation with retention features and restoring it with a material that is capable of hardening in situ. This process is called direct restorative dentistry because it is accomplished directly in the intraoral environment.Definitions Amalgam: Alloy of mercury with one or more metals. Amalgam alloy: Alloy which combines with mercury to form amalgam. Dental amalgam alloy: Alloy that is combined with mercury to form amalgam used for dental purposes. Amalgamation: Setting reaction of amalgam alloy with mercury. Trituration: The act of mixing amalgam alloy with mercury. Condensation: The process of placing the plastic amalgam mass into the tooth cavity and applying forces on it to adapt amalgam to the cavity walls. Burnishing: The act of smoothening the surface and margins of amalgam after condensation.
Dental Amalgam
Dental amalgam is a metal-like restorative material composed of a mixture of silver-tin-copper alloy and mercury. The unset mixture is pressed (condensed) into a specifically prepared undercut tooth form and contoured to restore the tooth's form and function. Once the material hardens, the tooth is functional again, restored with a silver-colored restoration. Amalgam has been the primary direct restorative material for over 150 years. It has been the subject of intense research but has been found to be safe and beneficial as a direct restorative material, and many people have benefited from amalgam restorations, which restore a tooth in a very economical manner because restorative alternatives would have been too expensive for many people. Amalgam restorations also can be bonded to tooth structure. When an amalgam restoration is bonded, many of the benefits of bonding accrue.
HISTORY
Amalgam was introduced to the United States in the 1830s. Initially, amalgam restorations were made by dentists filing silver coins and mixing the filings with mercury, creating a putty-like mass that was placed into the defective tooth. As knowledge increased and research intensified, major advancements in the formulation and use of amalgam occurred. However, concerns about mercury toxicity were expressed in many countries about the use of amalgam; concerns reached major proportions in the early 1990s. The American Dental Association has issued many statements expressing their support for the use and safety of amalgam as a restorative material.CURRENT STATUS
Today, the popularity of amalgam as a direct restorative material has decreased,'," in part because of concerns (valid or not) about its safety and environmental effects but primarily because of the recognized benefits and esthetics of composite as a restorative material. During the past 20 years, the number of amalgam restorations has decreased because of the concerns about the use of amalgam restorations relate to poor esthetics, weakening of the tooth by removal of more tooth structure, recurrent caries, and lack of adhesive bonding benefits (unless the amalgam restoration is bonded). Amalgam restorations are still well suited for restoring many defects in teeth. The ability to restore a tooth in a reasonably simple and economical manner has resulted in the continued use of amalgam by many dentists, even though most dentists have also increased their use of composite. The most significant factor that may eventually lead to an even greater reduction in amalgam use will most likely be concern about disposal of amalgam from the dental office. Increased attention to mercury contamination of water supplies has resulted in some communities passing regulations that have addressed this concern.Because of these environmental concerns about mercury contamination, the use of amalgam as a restorative material in many countries has already decreased.
Even with the concern about the disposal of mercury, many textbook advocates the continued use of amalgam as a direct restorative material. Research has demonstrated both the safety of the material and the success of restorations made from amalgam and is recognized as an excellent material for restoring many defects in teeth.
ADVANTAGES
Some of the advantages of amalgam restorations have already been stated, but the following list presents the primary reasons why amalgam restorations have been used successfully for many years:1. Ease of use and less technique sensitive
2. Excellent wear resistance3. Favorable long-term clinical research results
4. Lower cost than for composite restorations
5. Bonded amalgams have "bonding" benefits:
Less microleakage
Less interfacial staining
Slightly increased strength of remaining tooth structure
Minimal postoperative sensitivity
Some retention benefits
DISADVANTAGES
The following is a list of these and other disadvantages of amalgam restorations:
1. Noninsulating
2. Nonesthetic3. Less conservative (more removal of tooth structure during tooth preparation) and weakens tooth structure (unless bonded)
4. More technique sensitive if bonded
5. More difficult tooth preparation
6. Initial marginal leakage'
AMALGAM
Terminology:Amalgam: technically means an alloy of mercury (Hg) with any other metal.
Dental amalgam: is an alloy made by mixing mercury with a silver-tin dental amalgam alloy (Ag-Sn). In dentistry, it is common to use the term amalgam to mean dental amalgam.
Composition:
Silver (Ag)
Tin (Sn)
Copper (Cu)
Zinc (Zn)
With trace amount of indium (In) and palladium.
Classification.
The major approaches to classification of amalgams and the amalgam alloys on which they are based are in terms of (1) amalgam alloy particle geometry and size, (2) copper content, and (3) zinc content.
The classification of amalgam based on copper content is the main system in use today.
Classification according to Copper contents:
Low-Copper Amalgam. Low copper amalgams are often referred as conventional amalgams. A typical modern low-copper amalgam alloy may contain 69.4% Ag, 26.2% Sn, 3.6% Cu, and 0.8% Zn.
High-Copper Amalgam. High-copper amalgams are the materials predominantly used today. The increase in copper content to 12% or greater designates an amalgam as a high-copper type. High-copper materials can be either spherical or admixed in composition. A typical high-copper amalgam alloy may contain 60% Ag, 27% Sn, 13% Cu, and 0% Zn.
Class according to alloy particle geometry:
Spherical Amalgam (unicompositional): A spherical amalgam contains small, round alloy particles that are mixed with mercury to form the mass that is placed into the tooth preparation. Because of the shape of the particles, the material is condensed into the tooth preparation with little condensation pressure. This advantage is combined with its high early strength 25 to provide a material that is well suited for very large amalgam restorations, such as complex amalgams or foundations.Lathe cut (unicompositional): irregular shapes ranged from spindle to shavings.
Sheroidal (unicompositional): spherical but irregular surfaces.Admixed Amalgam: An admixed amalgam contains irregularly shaped and sized alloy particles, sometimes a mixture of spherical and lathe cut, which are mixed to form the mass that is placed into the tooth preparation. The irregular shape of many of the particles makes a mass that requires more condensation pressure (which many dentists prefer) and permits this heavier condensation pressure to assist in displacing matrix bands to more easily generate proximal contacts.
Class according to zinc contents:
Zinc containing alloyAlloys with more than 0.01% zinc (never exceed 1%). Zinc has been included in amalgam alloy as an aid in manufacturing by helping to produce clean, sound castings of the igots.
Zinc added to suppress oxidation. During manufacture it forms zinc-oxide film or layer covering the alloy and suppress oxidation of other elements. If moisture occur during amalgam application, delayed setting expansion may occur because zinc-oxide formation will elaborate hydrogen gas. Zinc containing amalgam has 20-50% longevity over zinc free. However, improved manufacturing procedures have resulted in elimination of zinc in most alloys.
Zinc free alloy
alloys with less than 0.01% zinc. Zinc free used for retrograde fillings due to difficult moisture control and danger of setting expansion at that part of the root.
New Amalgam Alloys. Because of the concern about mercury toxicity, many new compositions of amalgam are being promoted as mercury-free or low-mercury amalgam restorative materials. Alloys with gallium or indium or those using cold-welding techniques are presented as alternatives to mercury-containing amalgams.
The addition of indium (In) to Hg decreases the amount of Hg needed thus decreasing the amount of Hg vapor during and after setting. Unfortunately, none of these new alloys show sufficient promise to become a universal replacement for current amalgam materials.
Reaction of the alloy with the mercury:
The amalgam alloy is intimately mixed with liquid mercury to wet the surface of the particles so that a reaction will occur and lead to hardening of the mass. This mixing is called trituation.Low copper alloys:
Conventional amalgam sets by the reaction of Ag and Sn from Ag3-Sn(γ) gamma phase particles with mercury to produce two reaction product phases, an Ag2-Hg3(γ1) gamma one phase and a Sn-Hg(γ2) gamma two phase. These form solids and cause the mass to harden.
Ag3-Sn + Hg Ag2 Hg3 + Sn-Hg + Ag3-Sn
γ γ1 γ2 (unreacted)While crystals of the γ1 and γ2 phase are being formed the amalgam is relatively soft and easily condensable and carvable. As time progresses, more crystals of γ1 and γ2 are formed the amalgam becomes harder and stronger, and it is no longer condensable or carvable. The time between the end of the trituation and when the amalgam is hardens and no longer becomes workable is called working time.
Strength of phases:
γ = Strongest phase
γ1 = Intermediate phase
γ2 = Weakest phase
Because the original mixture contains a large excess of Ag3-Sn alloy particles, only a minor portion of the outside of the particles is consumed during the reaction with mercury. The unreacted portion of the original amalgam alloy particles remains as residual alloy particles, reinforcing the final structure. Reaction products (γ1 and γ2) form a matrix surrounding the residual (unreacted γ) alloy particles preventing the benefits of its strength so it is very important to minimize the amount of matrix that forms during the reaction.
Excess mercury lead to more reaction with gamma that will lead to production of larger percentage of γ1 and γ2 and this lead to weaken the amalgam.
Therefore after the reaction begins and the amalgam has been placed in the tooth preparation, it is important to compress (condense) the mixture to reduce voids in the material, adapt it closely to the tooth preparation walls, and express excess mercury-rich matrix. The mercury-rich matrix is removed from the surfaces of condensed material increments. This process ensures that the final structure is composed predominantly of reinforcing residual alloy within a minimum of reaction product matrix.
High copper alloys:
The advantage of the added copper is that it preferentially reacts with the tin and prohibits the formation of the weakest and most corrosive phase (gamma-two) within the amalgam mass. This change in composition reduces or eliminates possible deleterious corrosion effects on the restoration and the amalgam tend to have superior physical and mechanical properties. However, the reduction of the corrosion products reduces the formation of a corrosion layer at the amalgam-tooth interface, the formation of which sometimes aids in sealing the restoration.
The high copper amalgam is divided into two:
First: Admixed (High-Copper Amalgam): Alloy in (Ag-Cu) form. The mercury dissolved in the silver tin particles react as in low copper alloys and form the γ1 and γ2 phases, leaving some silver tin particles unreacted. In a relatively short time, however, the newly formed γ2 phase (Sn-Hg) around the silver tin particles reacts with silver –copper particles forming Cu6Sn5 the eta (η) phase of the copper tin system, along with some of the γ1 phase (Ag2Hg3) around the silver-copper particles. The reaction is simplified as follows:
Ag3Sn+Ag-Cu (eutectic)+Hg Ag2Hg3 + Sn-Hg + Ag3Sn + Ag-Cu γ γ1 γ2 γ(unreacted) (unreacted)
And later
Sn-Hg + Ag-Cu Cu6Sn5 + Ag2Hg3γ2 (eutectic) (η) γ1
Second: Sherical (High-Copper Amalgam): Alloy in (Cu3Sn) form. In this alloy the particles at the beginning of the reaction function like silver tin particles of the admix type, providing proper working time and ease of manipulation. Later the same particles function like the silver-copper particles of the admix type eliminating the γ 2 phase.
Ag3Sn + Cu3Sn + Hg Cu6Sn5 + Ag2Hg3
γ (η) γ1Corrosion:
When the setting reaction occurred, the material was subject to corrosion because a tin-mercury phase (γ2) formed (In low copper amalgams). This corrosion led to rapid breakdown of the amalgam restorations. Subsequent research for improving amalgam led to the development of high-copper amalgam materials which have low corrosion.Mechanism of corrosion:
(γ2) phase is the most corrosion susceptible product. Sn-Hg (γ2) reaction product crystals are long and bladelike, penetrating throughout the matrix. Although they constitute less than 10% of the final composition, they form a penetrating matrix because of intercrystalline contacts between the blades. This phase is prone to corrosion in clinical restorations, a process that proceeds from the outside of the amalgam, along the crystals, connecting to new crystals at intercrystalline contacts. This produces penetrating corrosion that generates a porous and spongy amalgam with minimal mechanical resistance.How to reduce corrosion:
The key feature of this degradation process is the corrosion-prone character of the Sn-Hg (γ2) phase which is eliminated by:
The use of more copper in the initial composition. High-copper amalgams set in a manner similar to low-copper amalgams except that Sn-Hg reactions are suppressed by the preferential formation of Cu-Sn phases instead. Cu-Sn phases that are part of the set amalgam matrix are much less corrosion-prone than the Sn-Hg phase they replace.
Express excess mercury-rich matrix. excess mercury can be excluded or controlled by:
Wringing the mass in a squeeze cloth.
Spherodizing the alloy decreasing the inter-particle spaces and require less force for condensation because of increased fluidity.
Good condensation (time, direction and force).
Use of precapsulated amalgam (mercury 42-45% by weight).
Types of Corrosion:
Both low-copper and high-copper amalgams undergo two kinds of corrosion, chemical corrosion and electro-chemical corrosion.
Chemical corrosion occurs most notably on the occlusal surface and produces a black Ag-S tarnish film. This reaction is limited to the surface and does not compromise any properties, except for esthetics.
Electrochemical corrosion is an important mechanism of amalgam corrosion and has the potential to occur virtually anywhere on or within a set amalgam. Electrochemical corrosion occurs whenever chemically different sites act as an anode and cathode. This requires that the sites be connected by an electrical circuit in the presence of an electrolyte, typically saliva. The anode corrodes, producing soluble and insoluble reaction products. If an amalgam is in direct contact with an adjacent metallic restoration such as a gold crown, the amalgam is the anode in the circuit. This type of electrochemical corrosion is called galvanic corrosion and is associated with the presence of different electrode sites.
Mechanism of seal of amalgam against microleakage
During electrochemical corrosion of low-copper amalgams, the Sn-Hg (γ2) phase is oxidized into Sn-O and/or Sn-O-Cl. The oxychloride species is soluble. The oxide (Sn-O) precipitates as crystals and tends to fill up the spaces along the margins of the amalgam, Sn-O helps seal the space against microleakage. Why space occur in amalgam? Amalgam has a linear coefficient of thermal expansion that is 2.5 times greater than tooth structure, and it does not bond to tooth structure (unless an amalgam bonding agent is used). Therefore, during expansion and contraction, spaces occur along the external walls.SHAPE \* MERGEFORMAT
Mechanical property of amalgamPrincipal mechanical properties of amalgam include compressive strength, tensile strength, and creep.
1: The compressive strengths: resistance to compression forces is the most favorable strength characteristic of amalgam.
The compressive strengths of high-copper amalgams are greater than those of low-copper amalgams because of the presence of the copper phases. High-copper amalgams have compressive strengths that range from 380 to 550 MPa (55,000 to 80,000 psi) and are very similar to those of enamel and dentin. Therefore dental manufacturers do not place much emphasis on increasing these values.
Factors affecting strength of Dental Amalgam
Particle size: Decreased size results in increased strength (due to increased surface area / unit volume)
Particle shape: Regular uniform shape result increased strength (due to more wettability, more coherent mass, less interrupted coherent interphases)
Microstructure of amalgam:
Increased γ and γ1 phases there is increased strength
presence of η phase there is increased strength
Increased γ2 phase, there is decreased strength
Porosities and voids in amalgam: Decreased strength. Formed due to:
Decreased trituration
Decreased condensation pressure
Irregularly shaped particles
Insertion of too large increments
Delayed insertion after trituration
Too less Hg (amalgam non-plastic)
Miscalculation of powder particle diameter to occupy available spaces
Hg/Alloy ratio: Increased Hg/Alloy ratio, decreased strength, because increased Hg results in
Decreased unreacted γ phase
Increased γ2 phase
Increased residual Hg (weakest phase) within amalgam
Trituration
Increased trituration within limits increases strength (due to increased coherence of matrix crystals).
Increased trituration beyond limits decreases strength ( due to cracking of formed crystals decreasing coherence).
Condensation pressure Increased pressure results in increased strength (due to removal of excess Hg within amalgam resulting in less residual Hg)
Temperature Amalgam loses 15% of its strength when its temperature is increased from room temperature to mouth temperature. It loses 50% of its strength when temperature is elevated beyond 60C (as in overjealous polishing).
Corrosion activity: Decreased corrosion activity results in increased adhesive integrity and therefore increased strength.
2: Tensile strength is important for fracture resistance. Both low and high-copper amalgams have low tensile strengths, but high-copper amalgam is lower overall. This is important because it is very likely that most intraoral loading conditions produce tensile stresses along the occlusal surface and at the margins. During direct contact by opponent teeth, cusps and/or amalgam restorations are stretched laterally, producing tension and perhaps flexion. Amalgams that are corroded or have inadequate bulk to distribute stresses may fracture. At margins, where amalgams are thinner, extrusion may have occurred, and corrosion may have compromised the integrity of the amalgam, fracture is even more likely. Amalgam is generally considered a brittle material. It is not capable of much plastic deformation before fracture when stressed at moderate-to-high strain rates, such as during vigorous chewing. Therefore traumatic stresses during chewing can produce fracture in an amalgam without sufficient bulk. Therefore the amalgam material must have sufficient bulk (usually 1 to 2 mm, depending on the position within the tooth) and a 90-degree or greater marginal configuration. In contrast, at slow strain rates such as expansion caused by phase changes or corrosion, amalgam (particularly low-copper amalgam) is capable of clinically significant plastic deformation (creep), even though the stresses are well below the elastic limit.
3: Amalgam creep is plastic deformation under a static load. Under a continued application of force in compression, an amalgam shows a continued deformation, even after the mass has completely set. The associated expansion makes the amalgam protrude from the tooth preparation. Such secondary expansion can occur throughout the clinical life of a restoration. On nonocclusal surfaces, the entire amalgam restoration may appear extruded, and this can produce unwanted esthetic problems or overhangs in some areas. On occlusal surfaces, abrasion and attrition tend to limit the overall extrusion. However, occlusal margins become fracture-susceptible ledges elevated above the natural contours of the adjacent enamel. Extrusion at margins is promoted by electrochemical corrosion, during which mercury from Sn-Hg rereacts with Ag-Sn particles and produces further expansion during the new reaction. This mechanism, called mercuroscopic expansion, was originally proposed as an explanation for the prevalence of marginal fracture associated with occlusal amalgams. While for high copper amalgams, corrosion products will be copper and tin oxides plus oxychlorides, but no mercury elaborates. So, no mercuroscopic expansion occurs at amalgam margins.
Factors affecting Creep
Microstructure of amalgam
Increased γ1 fraction, increased creep
Increased γ2 fraction, increased creep
Increased grain size of γ1, decreased creep
Presence of η phase, decreased creep
Hg/Alloy ratio Increased Hg/Alloy ratio, increased creep (due to more residual Hg)
Trituration
Overtrituration, increased creep
Undertrituration
Increased creep
Decreased creep in lathe-cut amalgam
Condensation pressure Increased pressure, decreased creep (due to less residual Hg)
Delay between trituration and condensation Increased creep
IMPORTANT PROPERTIES
The linear coefficient of thermal expansion (LCTE) of amalgam is 2.5 times greater than that of tooth structure, yet it is closer than the LCTE of composite.' Because amalgam is metallic in structure, it is also a good thermal conductor. Therefore an amalgam restoration should not be placed in close proximity to the pulpal tissues of the tooth without the use of a liner and/or base between the pulp and the amalgam.
AMALGAM RESTORATIONS
Amalgam functions as a direct restorative material by easily being inserted into a tooth preparation and, once hardened, restoring the tooth to proper form and function. Amalgam restorations may be bonded or nonbonded. Nonbonded amalgam restorations are still predominantly used, even though more bonded amalgam restorations are now being done. Both nonbonded and bonded amalgam restorations require a specific tooth preparation form into which the amalgam material is inserted. The tooth preparation form must not only remove the fault in the tooth and remove weakened tooth structure, but it also must be formed to allow the amalgam material to function properly. The required tooth preparation form must allow the amalgam to: (1) possess a uniform specified minimum thickness for strength, (2) produce a 90-degree amalgam angle (butt joint form) at the margin, and (3) be mechanically retained in the tooth. Without this preparation form, the amalgam could possibly be dislodged or fracture. After sealing the prepared tooth structure, mixing, inserting, carving, and finishing the amalgam is relatively fast and easy. For these reasons, it is a very userfriendly material that is less technique- or operator sensitive than composite.The use of bonded amalgam restorations has increased steadily. The mechanism of bonding an amalgam restoration is similar to that for bonding a composite restoration in some aspects, but it is different in others. For example, a bonded amalgam restoration, done properly, seals the prepared tooth structure and strengthens the remaining unprepared tooth structure. However, the retention gained by bonding may be minimal; consequently, bonded amalgam restorations still require the same tooth preparation retention form as nonbonded amalgam restorations. It should also be noted that isolation requirements for a bonded amalgam restoration are the same as for a composite restoration.
Factors Indicating the Use of Amalgam Restoration:
Occlusal Factors. Amalgam has somewhat greater wear resistance than composite. It therefore may be indicated in clinical situations that have heavy occlusal functioning. It also may be more appropriate when a restoration restores all of the occlusal contact for a tooth.
Isolation Factors. Unless an amalgam restoration is to be bonded, the isolation of the operating area is less critical than for a composite restoration. Minor contamination of an amalgam during the insertion procedure may not have as adverse an effect on the final restoration as the same contamination would produce for a composite restoration. However, if an amalgam restoration is to be bonded, the isolation needs are the same as for composite.
Operator Ability Factors. The tooth preparation for an amalgam restoration is very exacting. It requires a specific form with uniform depths and a precise marginal form. Many failures of amalgam restorations may be related to inappropriate tooth preparations. The insertion and finishing procedures for amalgam are much easier than for composite. However, if the amalgam restoration is to be bonded, the procedure is almost as demanding as that for a composite restoration.
Clinical Indications for Direct Amalgam Restorations.
Because amalgam is not an esthetic restorative material, it is no longer used much in areas where esthetics is critical. Because of the benefits of conservative bonded composite restorations, amalgam is also not used as much for the restoration of small defects on molars and premolars. However, because of its strength and ease of use, amalgam provides an excellent means for restoring large defects in nonesthetic areas, especially if it can be bonded. Generally amalgams can be used for the following clinical procedures:1. Moderate to large Class I and Class II restorations (especially including those with heavy occlusion, that cannot be isolated well, or that extend onto the root surface)
2. Class V restorations (including those that are not esthetically critical, cannot be well isolated, or are located entirely on the root surface)
3. Temporary caries control restorations (including those teeth that are badly broken down and require a subsequent assessment of pulpal health before a definitive restoration).
4. Foundations (including for badly broken-down teeth that will require increased retention and resistance form in anticipation of the subsequent placement of a crown or metallic onlay).
CONTRAINDICATIONS
Small lesionsEsthetic factors
Hyper sensitivity to amalgam
There are few indications for the use of amalgam for restorations in anterior teeth. Occasionally, a Class III amalgam restoration may be done if isolation problems exist. Likewise, in rare clinical situations, Class V amalgam restorations may be indicated in anterior areas where esthetics is not important. While esthetics is subject to wide variations in personal interpretation, most patients find the appearance of an amalgam restoration objectionable when compared to a composite restoration.
Therefore the use of amalgam in more prominent esthetic areas of the mouth is usually avoided. These areas include anterior teeth, premolars, and, in some patients, molars. Because the tooth preparation for an amalgam is larger than for a composite, most small to moderate defects in posterior teeth should be restored with composite rather than amalgam. Use of composite in these situations results in conservation of tooth structure.
CLINICAL TECHNIQUE
INITIAL CLINICAL PROCEDURES
A complete examination, diagnosis, and treatment plan must be finalized before the patient is scheduled for operative appointments (emergencies excepted). A brief review of the chart (including medical factors), treatment plan, and radiographs should precede each restorative procedure. At the beginning of each appointment, the dentist should also carefully examine the operating site and assess the occlusion, particularly of the tooth (teeth) scheduled for treatment.
Local Anesthesia. Local anesthesia may be advocated for many operative procedures. Profound anesthesia contributes to a comfortable and uninterrupted operation and usually results in a marked reduction in salivation. Because most amalgam tooth preparations are relatively more extensive, local anesthesia usually is necessary.
Isolation of the Operating Site. Isolation for amalgam restorations can be accomplished with a rubber dam or cotton rolls, with or without a retraction cord.
Also, a preoperative assessment of the occlusion should be made. This step should occur before rubber dam placement and identify not only the occlusal contacts of the tooth to be restored, but also those contacts on opposing and adjacent teeth. Knowing the preoperative location of occlusal contacts is important both in planning the restoration outline form and in establishing the proper occlusal contacts on the restoration. Remembering the location of contacts on adjacent teeth provides guidance in knowing when the restoration contacts have been correctly adjusted and positioned.
Matrix band Placement.
A matrix primarily is used when a proximal surface is to be restored. The objectives of a matrix are to:(1) Provide proper contact,
(2) Provide proper contour,
(3) confine the restorative material,
(4) Reduce the amount of excess material.
For a matrix to be effective, it should:
(1) Be easy to apply and remove,
(2) extend below the gingival margin,
(3) Extend above the marginal ridge height, and
(4) Resist deformation during material insertion.
In some clinical circumstances, a matrix may be necessary for Class I or Class V amalgam restorations.
It should also be noted that matrix application might be beneficial during tooth preparation to help protect the adjacent tooth from being damaged. The matrix, when used for this reason, would be placed on the adjacent tooth (teeth).
Matrix Band:
An amalgam matrix band for a typical inter proximal cavity preparations is approximately 2mm above the proposed marginal ridge height.
They are available in various thicknesses , the thinner band most often used , since the matrix itself is in intimate contact with the adjacent tooth , the thicker the band, the greater will be the difficulty in establishing proximal contact of the proposed restoration .
The band either :
Precut (ready made).
May also be cut from a ribbon.
Copper ring.
T-Band.
Stability of the matrix band is necessary during the insertion of the amalgam of its maximum value is to be obtained. Instability occurs most often as the loss of tooth tissue becomes greater. As less contain of the tooth remains to stabilize the desired portion of the matrix band, it becomes necessary to use copper ring.
Since the purpose of the proposed amalgam restoration is to return the involved tooth to proper health , function & form , concern must be given to contour the matrix band bucco lingual & occluso cervical directions , otherwise ; the gingival tissue in the area will be traumatized by food forced directly against them during mastication . A long standing irritation due to the trauma results in loss of the underlying bone support for the affected tooth, as well as for the adjacent.
Matrix band retainer:
A matrix band is retained in its selected position by a mechanical device called '' matrix retainer '', or by dental floss or compound.Retainers are classified into :
Ivory No.1 : surround the teeth from one surface , M or D.
Ivory No.8 : used when restoring M.O.D. cavity preparations. These retainers lie in the muco buccal fold allow the operation greater convenience.
Universal Matrix. The Universal matrix system (designed by B.R. Tofflemire) is ideally indicated when three surfaces (i.e., mesial, occlusal, distal) of a posterior tooth have been prepared. It is commonly used also for the two-surface Class II restoration. A definite advantage of the Tofflemire matrix retainer is that it may be positioned on the facial or lingual aspect of the tooth. Lingual positioning, however, requires the contra-angled design of the retainer (which can be used on the facial aspect as well. The retainer and band are generally stable when in place. The retainer is easily separated from the band to expedite removal of the band. Matrix bands of various occlusogingival widths are available. A small Tofflemire retainer is available for use with the primary dentition. Even though the Universal retainer is a versatile instrument, it still does not meet all the requirements of the ideal retainer and band. The conventional, flat Tofflemire matrix band (unburnished) must be shaped (i.e., burnished) to achieve proper contour and contact.
Compound-Supported Matrix. The compound supported matrix is rarely used, but it is an alternative to the Universal matrix. A wedged matrix supported by compound provides most of the essential qualities of a good matrix, especially when used for two-surface proximal restorations." It is more rigid than commercial matrices, provides better contact and contour, is virtually trouble-free during proper removal, requires very little proximal carving after the matrix removal, and only has one thickness of band material for which wedging must compensate.
Automatrix. The Automatrix (Dentsply Caulk, Milford, Delaware) is a retainerless matrix system with four types of bands that are designed to fit all teeth, regardless of circumference. The bands vary in height and supplied in two thicknesses. The indicated use of this matrix is for extensive Class II preparations, especially those replacing two or more cusps. As with all matrix systems, it has advantages and disadvantages. One advantage is that the auto-lock loop can be positioned either on the facial or lingual surface with equal ease. A disadvantage is that the bands are not precontoured, and development of physiologic proximal contours is difficult.
The Wedge:
A matrix band having been contoured & affixed in the retainer will be deficient in two aspects for the formation of an adequate restoration.First :
The force of condensation of each increment of amalgam will usually cause in excess contour at the cervical area.
Over extension of the restorative material in an apical direction , (overhang)
This resulting in an overhang can be avoided by wedging of the matrix band tightly against the tooth at the area just below the gingival floor .
The wedge must be trimmed to fit anatomically within the triangle formed by the matrix band , the proximal surface of the adjacent tooth , & the gingival tissue , usually placed lingually . Condensation against an unwedged matrix may cause the amalgam to extrude grossly beyond the gingival margin. (Obviously, without a wedge there will be some excess amalgam at the proximal margins that will be overcontoured, requiring correction by a suitable carver immediately after matrix removal.)
Second :
The wedging action between the teeth should provide enough separation to compensate for the thickness of the matrix band. This will ensure a positive contact relationship after the matrix is removed (after the condensation and initial carving of the amalgam).
The advantage (benefit) of the wedge is to:
Hold the band tightly against the gingival margin.Prevent an overhang of amalgam .
Provide sufficient separation of teeth for the thickness of the band material .
types of wedges :
wooden .plastic .
metal .
The effective force of the wedge is horizontally directed to the extreme cervical edge of the band, which has been extended approximately 1mm. beyond the cervical margin of the preparation.
If there is no wedge separation , it leads to :
A deficient proximal contact will likely result, food impaction between such teeth with light or lacking contact leading to loss of soft tissue & boney support which will occur after time.
Overhang : it is un desirable to establish a contact area with the adjacent tooth which will exhibit a resistance to the passage of dental floss equal to those of proximal contacts of the other teeth in that quadrant .
Loss the benefit of the band relaxation following wedge placement that will generally insure a good contact. This can be obtained by the loosing of the matrix retainer 1/4 turn to 1/2 turn.
When matrix & wedge are in place & the band should be burnished & pressed against the adjacent tooth , the gingival seal of the matrix should be tested by pressing a probe into the margin , if a defect is noticed , the wedge should be pressed more firmly into the inter space. Height of the matrix band above the adjacent tooth should be 2 mm. so that:
1. The band is more stable.
2. More convenient for condensation.
3. Helps in the correct judgment of the marginal ridge at this time.
Mixing (Triturating) the Amalgam Material
Manual: using mortar and pestleMechanical: using amalgamators
During the early part of the twentieth century, alloy powder and mercury were proportioned crudely and mixed manually.
DRAWBACKS OF MANUAL TECHNIQUE:
Mercury toxicity.Improper powder/liquid ratio.
Less mechanical and biological properties
Possible mixture contamination.
Somewhat complicated procedure.
Waste of time and material.
To proportion and mix amalgam more carefully, manufacturers later recommended the use of alloy pellets, mercury dispensers, reusable mixing capsules and pestles, and amalgamators.
Modern amalgams are produced from precapsulated alloy and mercury. The components are separated in the capsule by a special diaphragm that is broken when the capsule is "activated" just before mixing. Pre-capsulated (preproportioned) amalgam provides convenience and some degree of assurance that the materials will not be contaminated before use or an important spilled before mixing.
ADVANTAGES OF PRECAPSULATED TECHNIQUE:
Less mercury toxicity.
Proper powder/liquid ratio.
Best mechanical and biological properties.
Less chance for mixture contamination.
More time and material saving.
Easy procedure.
The manufacturer's directions should be followed when mixing the amalgam material. Both the speed and time of mix are factors in the setting reaction of the material. Alterations in either may cause changes in the properties of the inserted amalgam. After completing mixing the amalgam empty the triturated amalgam into a Dappen dish. It is not necessary to squeeze excess mercury (using an amalgam cloth) from the mix when using controlled mercury systems. Correctly mixed amalgam should not be dry and crumbly. It will have a minimal, yet sufficient, "wetness" to aid in achieving a homogeneous and well adapted restoration.
Effect of trituration on some amalgam properties:
Working time of all types of amalgam decreases with over-trituration.
Dimensional change: over-trituration results in slightly higher contraction for all types of alloys.
Compressive and tensile strengths are reduced in both over and under trituration of the spherical alloys.
Over-trituration increases creep, and under-trituration lowers it.
Inserting the Amalgam Amalgam is carried from dappen dish to the cavity using an amalgam carrier. It is important to properly condense the material into the tooth preparation. Lateral condensation (facially and lingually directed condensation) is very important in the proximal box portions of the preparation to ensure confluence of the amalgam with the margins.
The objectives of condensation of amalgam into prepared cavity are:
To secure adaptation of amalgam to walls & margins.
For further removal of the excess Hg.
To get compactness & homogeneity of amalgam with minimal amount of voids in the final restoration.
Spherical amalgam is more easily condensed than admixed (lathe-cut) amalgam, but some practitioners prefer the handling properties of the admixed type. Both types are easily inserted.
As a general rule, smaller amalgam condensers are used first. This allows the amalgam to be properly condensed into the internal line angles and secondary retention features. Subsequently, larger condensers are used.
If the amalgam is to be bonded, the adhesive application and amalgam condensation must occur simultaneously. This permits intermingling of the resin (which also is bonding to the tooth structure) with the amalgam particles. When bonding an amalgam restoration, usually the priming/ adhesive application for the prepared tooth structure is different than that for a composite restoration. The product manufacturer's directions must be followed. It is very important that the amalgam condensation occur before the adhesive polymerizes.
The condensation is started at the box till we fill the cavity, do over filling. At each stage the excess (Hg) should be removed, otherwise it may be trapped in the depths of the amalgam & constitute a weakness .
Condensation of amalgam that contains spherical particles requires larger condensers than are commonly used for admixed amalgam. Smaller condensers tend to penetrate a mass of spherical amalgam, resulting in little or no effective force to compact or adapt the amalgam within the preparation. In contrast, smaller condensers are indicated for the initial increments of admixed amalgam because it is more resistant to condensation pressure.
Almost all well-condensed amalgam will exhibit flush at the gingival margin , & this must be removed with a fine interproximal carver (with the exception of the gingival margin , it is desirable to leave a slight excess of material which will be removed during the polishing) .
Over filling : Put an excess of amalgam above the occlusal surface in order to:
Cover the cavo-surface margin completely to avoid exposure of those margins.
To be able to do proper carving .
To remove excess (Hg) .
Gross contouring of the occlusal (or initial carving) surface with the large spoon excavator & the approximation of the marginal ridge height which should be done before the removal of the matrix .
Burnishing
Objectives:To further decrease the size and number of voids.
To express excess Hg on the surface of the amalgam restoration.
To adapt amalgam to the cavosurface anatomy.
To condition surface of the amalgam for carving.
Carving the Amalgam.
Objectives:To produce a restoration with
No underhangs (no shouldering or shelving).
Proper physiological contours.
Minimal flash (no overhangs).
Functional, non-interfering occlusal anatomy.
Adequate, compatible marginal ridges.
Proper size, location, extent and inter-relationship of contact areas.
Physiologically compatible embrasures.
No interference with integrity of periodontium.
The amalgam material selected for the restoration has a specific setting time. The insertion (condensation) and carving of the material must occur before the material has hardened so much as to be noncarvable. Once pre-carve burnishing has been done, the remainder of the accessible restoration must be contoured to achieve proper form and function. A nonbonded amalgam is relatively easy to carve. However, a bonded amalgam is more difficult because the excess polymerized adhesive resin accumulates at the margins and is harder to remove. Care must be taken to prevent breaking out chunks of amalgam when carving a bonded amalgam.
A discoid-cleoid instrument is used to carve the occlusal surface of an amalgam restoration. The rounded end (discoid) is positioned on the unprepared enamel adjacent to the amalgam margin and pulled parallel to the margin. This removes any excess at the margin while not allowing the marginal amalgam to be overcarved (too much removed).
The pointed end (cleoid) can be used to define the primary grooves, pits, and cuspal inclines. However, an amalgam restoration is not carved with deep, acute grooves or pits because that may leave the adjacent amalgam material more subject to fracture.
For large Class II or foundation restorations, the initial carving of the occlusal surface should be rapid, concentrating primarily on the marginal ridge height and occlusal embrasure areas. The explorer tip is pulled along the inside of the matrix band, creating the occlusal embrasure form. When viewed from the facial or lingual, the embrasure form created should be identical to that of the adjacent tooth, assuming the adjacent tooth has appropriate contour. Likewise the height of the amalgam marginal ridge should be the same as that of the adjacent tooth. If both of these areas are developed properly, the potential for fracture of the marginal ridge area of the restoration is significantly reduced. Placing the initial carving emphasis on the occlusal areas for a large restoration permits the operator to more quickly remove the matrix and carve any extensive axial surfaces of the restoration, especially the interproximal areas. If the amalgam is too hardened to carve, may require the use of rotary instruments in the handpiece.
Once the initial occlusal carving has occurred, the matrix is removed to provide access to the other areas of the restoration that require carving.
All support for the band is withdrawn including the wedge , & the retainer . The proximal portion of the newly placed restoration should be removed at the band. The band should be withdrawing it lingua-occlusally, or bucco-occlusally. A circumferential band is first removed from the non-involved proximal side. Carving of the cervical margins should begin immediately following the removal of the band.
Finishing and polishing of the Amalgam Restoration.
Objectives:
To remove amalgam flash that has been left behind during carving.
To remove overhangs, major sometimes.
To correct minor enamel underhangs.
To convert superficial amalgam into a relatively inert layer galvanically, to decrease electrolytic corrosion.
To remove superficial scratches and irregularities: decreases fatigue failure, decreases concentration cell corrosion and decreases accumulation or adherence of plaque.
To make the restoration aesthetically more appealing.
Once the carving is completed, the restoration is visualized from all angles and an assessment of the thoroughness of the carving is made. Knowing the preoperative occlusal relationship of the restored and adjacent teeth is helpful in developing the appropriate contacts in the restoration; the tooth should be restored to appropriate occlusal contacts. Initially, the patient should be instructed to close very lightly, stopping when any contact is noted. At this point the operator should visually assess the occlusion. If spacing is seen between the adjacent teeth and their opposing teeth, the area of premature occlusal contact on the amalgam should be identified and relieved. Articulating paper is used to more precisely adjust the contacts until the proper occlusal relationship is generated. After the occlusion is adjusted, the discoid-cleoid can be used to smooth the accessible areas of the amalgam. A lightly moistened cotton pellet held in the operative pliers can be used to smooth the accessible parts of the restoration. If the carving and smoothing is done properly, no subsequent polishing of the restoration is needed and good long-term clinical performance will result.
A post carve burnishing is done by a light rubbing of the carved surface with a burnisher of suitable size & shape to:
Improve smoothness & produce a Satiny but not shiny appearance.
Produce a denser amalgam at the margins of occlusal preparations restored with conventional amalgam.
Polishing:
Amalgam surface should be polished to:Ensure minimal affinity for plaque.
Optimize corrosion resistance.
Better esthetic.
Polishing should be avoided before at least 24hrs of amalgam application. Various techniques used but mostly polishing rubber cups with pumice and adequate water coolant to avoid the hazards of over-heating amalgam surface (amalgam has low melting point).
Repairing an Amalgam Restoration.
If an amalgam restoration fractures during insertion, the defective area must be reprepared as if it were a small restoration. Appropriate depth and retention form must be generated, sometimes entirely within the existing amalgam restoration. If necessary, another matrix must be placed. A new mix of amalgam can be condensed directly into the defect and will adhere to the amalgam already present if no intermediary material has been placed between the two amalgams. Therefore the sealer material can be placed on any exposed dentin, but it should not be placed on the amalgam preparation walls. If the amalgam has been bonded, carefully condition and apply adhesive to the exposed tooth structure in the preparation.COMMON PROBLEMS: CAUSES AND POTENTIAL SOLUTIONS
The following is a list of common problems associated with amalgam restorations. It provides typical causes and potential solutions.
POSTOPERATIVE SENSITIVITY
Causes of postoperative sensitivity include:Lack of adequate condensation, especially lateral condensation in the proximal box.
Lack of proper dentinal sealing with sealer or bonding system.
Potential solutions include:
Proper condensation technique
Proper dentinal sealing technique
MARGINAL VOIDS
Causes of marginal voids include:Inadequate condensation
Material pulling away or breaking from the marginal area when carving bonded amalgam
Potential solutions include:
Proper condensation technique
Careful carving of marginal areas, especially bonded amalgam restorations
MARGINAL RIDGE FRACTURES
Causes of marginal ridge fractures:Axiopulpal line angle not rounded in Class II tooth preparations
Marginal ridge left too high
Occlusal embrasure form incorrect
Improper removal of matrix
Overzealous carving
Potential solutions include:
Proper rounding of axiopulpal line angles in Class II tooth preparations
Creating marginal ridge height correctly, with both the adjacent tooth and occlusion
Creating an occlusal embrasure form that mirrors the adjacent tooth
Clinical longevity CL50: is the median age for a group of related or similar restorations at which 50% of the restorations have been replaced due to clinical failure.
Clinical longevity CL50 for low copper amalgam 5-8 yrs.
Clinical longevity CL50 for high copper amalgam 24-25 yrs
Clinical Failure: is the point at which the restoration is no longer serviceable or at which time the restoration poses other severe risks if its not replaced. And these include:
Bulk fracture of restoration (especially at isthmus).
Corrosion and excessive marginal fracture.
Sensitivity or pain.
Secondary caries.
Fracture of the tooth.
AMALGAM RESTORATION SAFETY
Once reaction is complete, only extremely minute levels of mercury can release and those are far below the current health standard.In many cases the inconsistency in materials and techniques led to slow setting amalgams that release mercury from unset mass into unprotected dentinal tubules. Although there is no reported case of patient death, there were several cases of pulp death.
The health risk from dental amalgam use is greater for members of the dental office team than of the patient.
Metallic (elemental) mercury exist in liquid and vapor forms. As vapor can be inhaled and absorbed through the alveoli in the lung in 80% efficiency. This is clearly the major route of entry into human body. From skin about 0% efficiency, and 0.01% efficiency from GIT.
Once absorbed it has tendency to concentrate in certain organs like liver cause failure), kidney (nephropathy) and brain (Alzheimer disease). Then eventually all excreted depending on bodys ability to convert them into other forms
In the dental office, the sources of mercury exposure related to amalgam include:
(1) amalgam raw materials being stored for use (usually as precapsulated packages);
(2) mixed but unhardened amalgam during trituration, insertion, and intraoral hardening;
(3) amalgam scrap that has insufficient alloy to completely consume the mercury present;
(4) amalgam undergoing finishing and polishing operations; and
(5) amalgam restorations being removed.
It is difficult, if not impossible, to totally contain liquid or gaseous mercury because it is very mobile, has a high diffusion rate, and penetrates through extremely fine spaces. Even in packages that include plastic blister wrapping and layers of cardboard, mercury vapor leakage is possible. Therefore mercury-containing products should not be stored in the open, but rather in closets or cabinets, to minimize local concentrations in the rest of the offices. Storage locations should be near a vent that exhausts air out of the building.
Dental mercury hygiene recommendations
Ventilation: fresh air exchanges and periodic replacement of filters as it may act as trap for mercury.
Monitor office: mercury vapor level can be monitored by dosimeter badges. Allowed level is 25(ADA) microgram (g)/m3.
Monitor dental personnel: average urine mercury level 6.1g/litre.
Office design: proper design facilitating spill contaminants and cleanup.
Use of precapsulated alloy. Otherwise use unbreakable containers.
Use amalgamator fitted with a cover.
Handling care: usually try mercury no-touch techniques.
Use high volume evacuation while removing or finishing amalgams.
Use mask when dealing with amalgam.
Dispose mercury contaminated items, and carefully clean sterilizable instrument prior to heat sterilization.
Cleanup spilled mercury droplets properly by using trap bottles, tapes, or freshly mixed amalgam to pick up droplets.
Clothing: only wear professional clothing in the dental operatory.
SPHERICAL OR ADMIXED AMALGAM
As presented earlier, both spherical and admixed amalgam types have beneficial qualities. Regardless of the advantage of one type versus the other, the choice usually is made by the dentist's preference on the handling characteristics of the material. Still, spherical materials have advantages in providing higher earlier strength and permitting the use of less pressure. This advantage provide a material that is well suited for very large amalgam restorations, such as complex amalgams or foundations. Admixed materials, on the other hand, permit easier proximal contact development because of higher condensation forces.BONDED AMALGAM RESTORATIONS
Bonded amalgams are indicated for large restorations that require additional retention features or strengthening of the remaining unprepared tooth structure. However, the amount of increased retention gained from bonding an amalgam is controversial. While recognizing some increase in retention is gained from bonding an amalgam, but its preferred to use a typical secondary retention form preparation features (e.g., grooves, locks, pins, slots) for large amalgam restorations that are bonded.Small to moderate amalgam restorations do not require bonding; in fact, many small to moderate restorations would be better indicated for composite restorations.
Disadvantages of amalgam bonding:
Incorporation of large amount of bonding within amalgam may adversely affecting its mechanical properties. So, care should be taken during amalgam bonding procedures.
Expensive.
Technique sensitive.
Complicated procedure and time consuming.
Differences in coefficient of thermal expansion between tooth, bonding, amalgam interfaces may lead to bond failure.
Gingival displacement of bonding may cause gingival irritations.
Lack of long term data to support its validity.